In a typical networking environment used for routing data, the environment may include a number of Customer Edge (CE) routers, a number of Provider Edge (PE) routers and a packet-switched network (PSN). Data, encapsulated in layer-2 frames, may be forwarded from a first CE router to a first PE router, from the first PE router across the PSN to a second PE router, and from the second PE router to a second CE router. A Pseudowire (PW) may be utilized to transfer data across the PSN. A Pseudowire is a mechanism that emulates attributes of a service such as Asynchronous Transfer Mode (ATM), Frame Relay (FR), Point-to-Point Protocol (PPP), High Level Data Link Control (HDLC), Synchronous Optical Network (SONET) Frames or Ethernet over a PSN. The functions provided by the PW include encapsulating Protocol Data Units (PDUs) arriving at an ingress port, carrying them across a path or tunnel, managing their timing and order, and any other operations required to emulate the behavior and characteristics of the particular service. In a particular embodiment, PWs are used to carry ingress layer-2 traffic from an ingress PE router to an egress PE router, and then forward the layer-2 traffic out of an egress port of the egress PE router.
The environment further includes an attachment circuit (AC) which comprises the physical or virtual circuit attaching a CE to a PE. An AC can be a Frame Relay PVC identified by a local Data Link Connection Identifier (DLCI), an ATM PVC identified by a Virtual Path Identifier/Virtual Channel Identifier (VPI/VCI), an ATM port, an Ethernet port, a Virtual Local Area Network (VLAN), a High Level Data Link Control (HDLC) link, a PPP connection on a physical interface, a PPP session from an Layer-2 Tunnel Protocol (L2TP) tunnel, or similar such element.
Reference is made herein to layer-2 and layer-3 devices and data. These terms refer to the standard model for networking protocols and distributed applications, the International Standard Organizations Open System Interconnect (ISO/OSI) model. This model defines seven network layers; a physical layer, a data link layer, a network layer, a transport layer, a session layer, a presentation layer and an application layer.
Layer 2 is referred to as the data link layer and assigns the format of data on the network. The data link layer handles the physical and logical connections to the packet's destination using a network interface. A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world.
Layer 3 is known as the network layer. Internet Protocol (IP) version 4 or version 6 uses the network layer as a network layer interface. IP version 4 identifies each host with a 32-bit IP address. IP addresses are written as four “dot-separated” decimal numbers between 0 and 255. Some part of the IP address identifies the network and the remaining bits identify a particular host on that network.
In conventional systems, a PDU (e.g., a frame) traverses the networking environment beginning at a first CE router and ending up at a second CE router. The first CE router sends a layer-2 PDU to an ingress PE router. The ingress PE router receives the PDU and encapsulates the PDU with MPLS labels which are used to identify the individual port/circuit and the egress layer-3 PE router. MPLS labels are the key forwarding mechanisms in an MPLS network.
The encapsulated PDU is then forwarded on the PW, across the packet-switched network, to an egress layer-3 PE router. The egress layer-3 PE router removes the MPLS label that identifies the port/circuit that was added by the ingress PE router and forwards the layer-2 PDU to the second CE router.